The background to the invention lies in a different field. In our British Patent Application published under no. 2 236 887 (which is incorporated herein by reference) there are disclosed rotatable mechanical coin escrow mechanisms. These are intended to be driven by an electric motor which rotates the drum of the mechanism by an integral number of complete rotations so as to cause coins selectively to be delivered to a cashbox, or returned to a user. The number of coins present in the escrow mechanism when it is to be driven varies greatly, and so therefore does the inertia of the mechanism, this being rather small when only the empty plastic drum has to be rotated but two or more times as great when the drum is carrying its maximum number of coins. The amount of friction to be overcome, both in the bearings of the drum and between the coins and the drum, also varies considerably.
A feasible, and conventional, system for rotating the drum would be a small high-speed DC electric motor driving the drum through a large reduction ratio gearbox, in conjunction with an indexing system such as an optically-sensed tooth wheel, for the purpose of detecting absolute drum position.
Low cost is an essential requirement in the applications for which such escrow mechanisms are intended, such as pay telephones, and a further requirement in that context is for the drive power to be derived from the telephone line.
Cheap, high-speed (say 3000 rpm) DC electric motors do not have sophisticated and therefore relatively expensive very low friction bearings, and relatively high frictional losses are also characteristic of cheap high reduction ratio gearboxes. It is therefore very difficult to produce cheap drive systems of the above kind which will have a sufficiently low power consumption (typically less than 100 mW) to permit their operation from telephone lines.